The present invention relates to a method for generating a tripping signal as a function of the magnitude and the duration of an overcurrent, in which a measurement variable which is derived from the overcurrent is rectified and sampled, the sampled measurement values are converted into corresponding digital values, the digital values of a given function are weighted accordingly and summed, the tripping signal is formed at a sum value above a given reference value, and the summing is cancelled before the tripping signal is formed if the overcurrent falls below a given threshold for a certain time.
A known monitoring equipment operates according to such a method which is described in DE-OS No. 29 50 031. See also U.S. Pat. No. 4,219,860 or G.B. No. 2,037,104. In this known monitoring equipment, a measurement variable obtained by rectification from the current to be monitored is sampled by means of a sampling circuit and the sampling values so obtained are converted into corresponding digital values in an A/D converter. The digital values are weighted in a function generator and are added up after being weighted in an adding circuit. If the content of the adding circuit reaches a predetermined reference value, a tripping signal is generated. In the known monitoring equipment, a circuit module is connected to the A/D converter, by which the current is continuously checked to determine if it has overcurrent values. As long as this is the case, a clock-controlled counter which is connected to the circuit module is continuously reset. If the resetting signal of the circuit module fails to arrive (the current is below overcurrent values), the clock-controlled counter runs up to a preset counter reading and then resets the adding circuit. So that resetting of the adding circuit does not occur merely because the instantaneous values of the waveform of the rectified measurement variable fall below a value characterized as an overcurrent, the counter must be designed in the known monitoring equipment in such a manner that this predetermined counter reading can be reached only after a time which is longer than the duration of a half-wave of the current to be monitored. This can lead to a situation that in an isolated case, tripping still takes place without a need for it. In any case, the known monitoring equipment operating according to the method described for currents of 50 or 60 Hz requires counters set differently with respect to the critical counter readings.